This invention relates to an improved process for flocculating and thickening and/or dewatering aqueous mineral slurries containing 10% or more by weight of solids. More particularly, this invention relates to the use of a polymeric flocculant prepared by the reaction of dicyandiamide and formaldehyde for treating finely divided mineral slurries such as kaolinitic clays, calcium carbonate, calcium sulfate and other finely divided minerals, to filter or otherwise dewater them prior to shipment of the minerals. This invention also relates to the use of the aforementioned polymeric flocculants in combination with polymers containing 2-acrylamido-2-methyl propyl sulfonic acid monomers described in my U.S. Pat. Nos. 4,647,382 and 4,471,838.
Flocculants are reagents which are added to suspensions of solids to cause the solids to agglomerate (or flocculate) into larger particle sizes which settle and/or dewater more efficiently than in their original state. Polymeric water-soluble flocculants may be nonionic, anionic, cationic or amphoteric. The particular type of flocculant to choose in a given application may depend upon the nature of the surface of the suspended solids, the pH of the aqueous slurry, and the desirable or undesirable side effects.
Approximately 5 to 6 million tons of kaolinitic clay are mined, processed and shipped from the southeastern United States per year. The clay is usually mined by the open pit method. It is then slurried in water at about 20-30% solids, dispersed with various combinations of inorganic and organic dispersants to maintain a low viscosity, and processed. Processing includes removal of iron and other magnetic impurities, removal of sand and silt, bleaching for whiteness and brightness, and, finally, dewatering. Dewatering is commonly accomplished by rotary vacuum filters although other methods of dewatering such as pre-thickening, centrifugation, and filtration on pressure filters are feasible. The filter cake obtained from the rotary vacuum filters is, in the present state of the art, usually about 50 to about 60% solids.
Also in the present state of the art, a portion of the filter cake is typically spray dried and blended back with the remaining filter cake to obtain an aqueous slurry containing about 70% solids or more. Additional dispersants may be added to the 70% solid slurry with mixing as needed to obtain a final Brookfield viscosity of less than 1000 cps., and preferably less than 500 cps. This low viscosity, 70% solids slurry is then shipped in bulk to the customer.
My U.S. Pat. Nos. 4,647,382 and 4,741,838 disclose the use of certain anionic polymers of 2-acrylamido-2-methyl propane sulfonic acid, as flocculants which improve the dewatering of mineral slurries and which also enable the ready redispersal of the filter cake to a low viscosity slurry.
While certain organic cationic polymers are known to effectively flocculate pigment slurries, specifically kaolinitic clays, calcium carbonate and calcium sulfate, these products have not been employed commercially in dewatering pigment slurries. The cationic polymers have not been commercially adopted because during subsequent re-dispersal to a high solids, low viscosity slurry the floc formed by state-of-the-art organic cationic polymers will not break down. In fact it has been impossible to satisfactorily redisperse mineral slurries so treated by conventional means.
U.S. Pat. No. 4,217,209 (see particularly col 11, line 53) proposes the use of polymers of dicyandiamide, formaldehyde and ammonium sulfate to remove heavy metal sulfides from dye wastes. It is noteworthy that the above is the only condensation polymer in the list of cationic polymers proposed in Table 3--all the other cationic polymers are chain growth polymers. Even more worthy of note is that there are no performance results reported in '209 for the dicyandiamide formaldehyde and ammonium sulfate polymers, nor are these products included in the claims.
The '209 process is a very specific, multi-step process wherein the waste stream is pretreated with a substantial amount of an absorbent filter aid such as activated charcoal or Celite followed by treatment with a high molecular weight anionic polymer and finally a treatment with a cationic polymer. By contrast the present invention involves a one-step process of adding the cationic polymers described herein to a mineral process stream to obtain flocculation, which may be followed by addition of a particular (AMPS-containing) amionic polymer.
U.S. Pat. No. 4,217,209 is concerned with forming the insoluble sulfides of heavy metals which have an initial concentration of 100-5000ppm in the waste stream and subsequently removing them by flocculation. In further contrast, the instant invention is directed to more efficient dewatering of mineral slurries which contain 10% (100,000 ppm) or more solids prior to dewatering and as high as 50% (500,000 ppm) or more solids after dewatering.
Moreover, the concentrated solids resulting from the practice of the art in '209 is a waste and is simply disposed of in a safe manner. The concentrated solids resulting from the practice of the instant invention, conversely, are a product to be sold commercially. The high level of absorbent material which is an integral part of the '209 process would certainly be detrimental to product quality. Celite would without doubt lower the brightness, etc. of the mineral products of the present invention; the activated charcoal being black would surely cause the white mineral products to be unsaleable.
Finally, the concentrated solids resulting from the instant invention (usually 50-60% solids) are normally supplemented with dry mineral to bring the solids to 60-70% which must then be redispersed to a low-viscosity (usually less than 500 cps) slurry, There is no indication the concentrated solids from '209 are even near the 50% or more normally obtained in the practice of the present invention, nor is there any reason or need within the teachings of '209 to add solids to increase their concentration.
U.S. Pat. No. 4,271,028 discloses the use of condensation polymers of dicyandiamide with formaldehyde and related polymers for removal of protein type material from waste streams. However, upon reading '028, it is obvious that the dosages required to obtain satisfactory flocculation are enormous: 3000 to 6000 ppm based on the total liquid waste stream. If one calculates these dosages on a lbs./ton basis, based on example 1 of '028 the dosage range is 3600 to 7200 lbs. (active) of the condensation polymer per ton of influent solids. Comparatively, the dosages of the condensation polymer for purposes described herein are 0.5 to 250 ppm based on the total liquid sample (wherein the solids are 10%) or 0.01 to 5.0 lbs./ton. Differences of this magnitude are clearly indicative of patenably distinct processes. Further, the dosages necessary to practice the teachings of '028 are totally beyond the realm of acceptability from a practical, technical or economic standpoint for the use described in the instant invention.
Second, the use of a multivalent metal ion as a co-coagulant is necessary in order to successfully practice the process described in the '028 patent. In examples 1 and 3, an electrolytic process which utilizes a sacrificial electrode to provide Al.sup.+3 ions is an integral part of the treatment process. Example 3 clearly states that the Al.sup.+3 ion is necessary to obtain good flocculation. In example 2, the only example which does not include the Al.sup.+3 producing step, the multivalent nickel ion is already contained in the waste stream being treated. Further, although the dosage of Al.sup.+3 ions in Examples 1 and 3 cannot be calculated from the available data, the 1600 ppm of nickel ion in Example 2 strongly indicated that the necessary dosage of multivalent metal ion is also extremely high. Even further, the claims are written to a process which includes the electrolytic process.
Finally, there is again no need to prepare a high solids, low viscosity slurry from the "dewatered" solids. There is also no reason to anticipate that such would be possible if one attempted to do so. In fact, the solids in any slurry prepared from the "dewatered" solids of '028 would be predominently polymer and aluminum or nickel compounds.
Further, the prior art specific to use of the condensation polymers of dicyandiamide and formaldehyde gives no indication that these products would be effective flocculants or thickeners of any materials at ordinary dosages or in the absence of large dosages of an inorganic coagulant. In fact, the dosages employed in examples of the prior art showing these polymers were so extremely high as to actually teach away from consideration of these products as flocculants for mineral slurries, where large volumes of slurries are treated.